Composite polymer structures are produced in a broad range of commercial settings. Often, a composite is formed by a polymerization reaction in which reactants are mixed together and then cured, for example, by the addition of catalyst and/or heat to promote the polymerization reactions. Proper curing is extremely important as the structure and properties of polymers strongly depend on the extent of cure.
One area where the monitoring of the cure of polymerization reactions is particularly important is in the area of polymer coatings. Often a large roll of substrate in the form of a web must be thoroughly and evenly coated with a polymer. The web of substrate is generally withdrawn from the roll and coated with a mixture of the appropriate reactants and catalyst as necessary for promoting the particular polymerization reaction. Such techniques are well-known in the art. The coated web is cured such as by passing it through a curing oven. The cured web is then either trimmed into final products or rolled up again for later use or further processing.
Silicone release liners are a class of coated substrate commonly used to protect pressure-sensitive adhesives and prevent the adhesive from inadvertently bonding to surfaces prior to application. Release liners are manufactured by applying a release coating onto a substrate or backing film, normally of paper. Various silicone compositions have long been used for the release coating. Certain release coatings are applied by dispersing the silicone composition in a solvent in order to reduce the viscosity to a point where the composition can be easily coated over the substrate. The release liner is then cured by heating to drive off the solvent and cause the silicone composition to crosslink. A pressure sensitive adhesive can then be applied to the cured release liner. The adhesive forms a limited bond to the resulting cross-linked silicone layer of the release liner.
Other release coatings use silicone compositions, such as polydimethylsiloxanes which have a sufficiently low viscosity to allow their application to a substrate without use of solvents. These solventless or 100% solid release coatings are mixed with a reactive cross-linker and cured by a Group VIII precious-metal catalyst, such as platinum. Upon application of heat, the catalyst promotes cross-linking of the silicone polymers, curing the release coating so that acceptable release properties are achieved.
Both solvent-applied and 100% solids release coating are able to protect a pressure sensitive adhesive until use because the adhesive forms a limited bond to the cross-linked silicone layer of the release liner. The release properties of the release liner can be modified by varying the cross-link density of the cured release coating. This occurs prior to application of the release liner to the pressure sensitive adhesive.
Release coatings are commercially manufactured in large quantities by using coating machines where large rolls of backing film, such as paper, are fed to a coating head where the release coating is applied. The coating is cured by passing the coated film through an oven. Proper cure is achieved by controlling either oven temperature or residence time of the coated film through the oven. Improper coating or cure of the release liner can adversely impact the performance of the pressure-sensitive adhesive. If the release coating cure is not complete, weld of the adhesive to the release liner can occur and the silicone can transfer to the pressure sensitive adhesive and adversely affect adhesive properties. On the other hand, overcure of the release liner may not adversely affect the release liner's properties, but will result in unnecessary expense.
Chapter 24 of the Handbook of Pressure Sensitive Adhesive Technology, Second Edition (1989), Edited by D. Satus provides an excellent summary of silicone release coating technology, and is incorporated herein by reference.
Currently, several methods exist for measuring whether proper cure of release liners has been achieved. Most of these methods, however, require a sample of the release liner, and cannot be performed while the coating apparatus is on-line. As a result, entire runs of release liner may need to be scrapped because an insufficient cure was not detected in time to adjust the product in process.
U.S. Pat. No. 5,107,008 to Revis et al. relates to curable fluorescent organopolysiloxane release coating compositions. These compounds when incorporated into the release coating provide fluorescence when exposed to ultraviolet light, which is different, and detectable apart, from the fluorescence that is produced by the paper-whitening components used in the paper industry. The fluorescence emission spectra of dansyl functional siloxanes compositions of that invention show a single emission wavelength maximum of 465 nanometers. There is no indication that those compositions exhibit dimer or excimer formation making those compositions inoperable under the instant invention. While the exposure to ultraviolet light of the cured coating is suggested as having utility for measuring coating thickness by measuring coating fluorescence, there is no suggestion regarding determination of degree of cure of the compositions on a substrate.
One on-line method of monitoring cure was recently disclosed in U.S. Pat. No. 5,047,444 to Devoe et al. This patent discloses the use of a latent fluorophore that is added to the release coating as a cure monitor. When subjected to the cure conditions, this cure monitor forms an ultraviolet detectable fluorophore. The cure is monitored by applying an ultraviolet light source of a particular wavelength to the release liner. The ultraviolet radiation is absorbed by the fluorophore which in turn emits radiation which can be detected by photosensing apparatus. The intensity of the emission can be used to determine whether proper cure has been achieved.
Despite being an improvement over the prior methods of cure determination, this method has certain deficiencies. For example, this method requires that the cure monitor be selected so that it has a comparable reaction rate to the cure rate of the release coating. Furthermore, the fluorophores added to the release coating as cure monitors often have very different physical properties from those of the silicone compositions in the release coating. Silicone compositions are generally nonpolar molecules with low surface energy. Fluorophores, on the other hand, can be polar molecules with high surface energy. The differences in properties can lead to phase separation between the additive and the silicone compositions, causing skips in the coating application resulting in a release that is too tight, or otherwise incorrect determinations of cure.
Another on-line method of monitoring cure of polymers in general is disclosed in U.S. Pat. No. 4,651,011 to Ors et al. In this method, non reactive fluorophores are added to the polymer system to be cured. The polymer is exposed to polarized radiation of a wavelength that will excite the fluorophores causing them to fluoresce. The fluorescent emission is measured at two predetermined angles relative to the exciting radiation. Typically, one measurement is taken from an angle parallel to the exciting radiation and the other is taken perpendicular to the exciting radiation. These measurements indicate the orientation distribution of the fluorescent molecules in the polymer matrix. As the polymer cures and cross-linking proceeds, the fluorophores tend to assume some preferred orientation direction. The equation: ##EQU1## where I.sub.0 and I.sub.90 are the fluorescent intensities of the parallel and perpendicular emissions respectively can be used to determine the orientation distribution, r. The orientation distribution is in turn used to determine the cure of the release coating.
This method also has deficiencies in that the physical properties of the added fluorophore may be incompatible with those of the polymer matrix. This can cause phase separation between the components, or migration of the additive into the backing film when the method is used on release coatings. The result is undesirable release performance due to skips in the coating or incorrect determination of cure.
This invention is directed to the determination of the degree of cure in silicone release liner and other polymer coated substrate.